A Fluidised- bed ow
B Fluidised- bed ow
C Fluidised- bed ow
Packed- bed flow
Packed- bed flow
∆P
∆P
Pressure
Pressure
Pressure
Figure 2 Pressure profiles in standpipes
one of two methods. The first is a high-pressure gas source with a control valve feeding the fluidisation taps, which have upstream restriction orifices regulating the flow evenly to the taps. As shown in Figure 1 , this system relies on sonic velocities through the orifices to regulate the aeration. If the supply gas pressure drops below the minimum requirement for this system, the flow to the individual taps will become uneven. Plugged taps can also cause problems. The second method uses individual flow indicators and controllers that allow each tap to operate independently of the others. This costs more but gives greater control of the unit. Either plant air or superheated steam can be used in the unit for aeration. These streams must be dry. Any water entering the unit will flash, causing pressure surges and irregular catalyst flow. Wet steam going anywhere into the unit can cause pressure surges and possibly damage the metallurgy and refractory. A single gauge pressure survey can locate where the defluidisation is occurring in the standpipe. Flow rate adjustments to the taps should be based on the expected direction of the gas being injected to improve flow. Figure 2 shows the desired pressure profile down the standpipe and the result of defluidisation. If catalyst circulation changes significantly, the catalyst may be over-aerated. Small bubbles may coalesce, forming larger bubbles, reducing the density ( r ) of the catalyst in the standpipe. If the bubbles get large enough, they may par- tially block catalyst flow. The rapid build-up and breaking of these bubbles may cause rapid slide valve (and temper- ature) fluctuations. Additional start-up and turnaround concerns At design conditions, the catalyst typically circulates at 5-6 ft/sec down the standpipes, while the bubbles want to rise between 1-3 ft/sec, depending on their size. While a 6-inch bubble is the reported maximum stable bubble size for cracking catalysts, it is possible that bubbles can reach the size of the standpipe if the bubbles become stagnant. This type of behaviour occurs when the feed rate is low during start-ups or turndowns.
If the catalyst is flowing down at 3 ft/sec, it is inevitable that the coalescence of the aeration gas into large bubbles will result in unstable catalyst flow. On start-ups, the solu- tion is to get the feed rate high enough to avoid this phe- nomenon. When troubleshooting a cat circulation problem, calculate the velocity of the catalyst going down the stand- pipe to ensure it is above the bubble rise velocities. The formula for the amount of aeration gas to be added to the standpipe was derived by Zenz:
Minimum ACFM @PSIA = 2,000 (T/M) P BSP 1 – 1 – 1 – 1 P TSP r MF r P r TSP r P
Where: T/M = tons/minute catalyst circulation P TSP = Pressure at top of standpipe P BSP = Pressure at bottom of standpipe r MF = Density at minimum fluidisation r tsp = Density at top of standpipe r p = Density catalyst particle
The particle density ( r p ) is a function of the catalyst’s total pore volume and skeletal density and is the same density used in cyclone calculations. Frequently, too much aera- tion is added, causing circulation issues and reducing the amount of catalyst in a fixed volume, which manifests itself as a low density in the standpipe. This reduces the pressure above the regenerated catalyst slide valve. The equations for the amount of aeration gas to add tend to overestimate the amount needed, and many refiners use about 70% of this value. The overall pressure control of the unit also allows some flexibility for control of the regenerated catalyst slide valve pressure drop. Figure 3 shows the control scheme used for most FCC units and the basic pressure balance for a side-by-side design. The pressure above and below the slide valve determines the pressure drop across the valve. The pressure above the slide valve is the regenerator pres- sure; plus the height of the catalyst above the standpipe inlet; times the bed density; plus the vertical height of the
17
Revamps 2023
www.digitalrefining.com
Powered by FlippingBook